Dehydration and thermal stability of elpidite: An in-situ single crystal X-ray diffraction study

Elpidite Na2ZrSi6O16 center dot 3H(2)O is a natural zirconosilicate with space group Pbcm. A sample from Mt. Saint Hilaire (Canada) was investigated between 25 and 350 degrees C under different humidity conditions (RH = 0% and at RH = 100%). The largest structural channels are eight- and ten-rings delimited by corner-sharing SiO4 tetrahedra and ZrO6 octahedra. Under dry conditions, the structure started liberating water at 75 degrees C. At 100 degrees C, when it lost one H2O molecule, elpidite underwent a structural change from space group Pbcm to Cmce. The structure continuously released water up to 225 degrees C until it became anhydrous. From 225 degrees C to 350 degrees C no structural changes were observed. Associated with the transition from Pbcm to Cmce symmetry, the unit-cell volume reduced by 2.7%. An additional decrease is observed up to 225 degrees C when the contraction reached 3.4% of the starting volume. After release of all H2O molecules, increase of temperature led to gradual expansion of the unit cell from 2940.79 (7) angstrom(3) at 225 degrees C to 2947.4 (5) angstrom(3) at 350 degrees C. Supplementary ex-situ heating experiments showed that the structure was retained up to 950 degrees C but after heating at 1200 degrees C the sample melted. Under humid conditions a dehydration-trend similar to that found under a dry N-2-atmosphere was observed but the structural changes occurred at slightly higher temperatures. Immersion of a dehydrated sample in water for 30 days led to only marginal rehydration and the Cmce symmetry was preserved. (C) 2016 Elsevier Inc. All rights reserved.